The invention concerns a method of actuating a device for varying the valve timing of an internal combustion engine, which method can be advantageously implemented particularly in a camshaft adjusting device having a hydraulically releasable start locking.
Such a method relates particularly to camshaft adjusting devices of the generic type disclosed in DE-OS 197 26 300. These devices, generally designated in the technical field as axial piston or rotary piston adjusting devices, are generally configured as hydraulic adjusting drives that comprise an element fixed in driving relationship to the crankshaft of the internal combustion engine and an element rotationally fixed to the camshaft. The element fixed to the crankshaft is in power-transmitting relationship with the element fixed to the camshaft through at least one hydraulic working chamber configured within the device, each hydraulic working chamber being divided by an adjusting element within the device into a so-called A pressure chamber and a B pressure chamber. A pivoting or fixing of the element fixed to the camshaft relative to the element fixed to the crankshaft is effected by a selective or a simultaneous pressurizing of the A and/or B pressure chamber of each hydraulic working chamber, the pressurizing of the pressure chambers being controlled by an electromagnetically actuated valve piston of a hydraulic valve whose solenoid is actuated by a microprocessor as a function of different operation parameters of the internal combustion engine. Normally, this valve piston of the hydraulic valve permits, in a non-energized or low energized state of the solenoid, a pressurization of the B pressure chamber of each hydraulic working chamber, in a high or maximum energized state of the solenoid, a pressurization of the A pressure chamber of each hydraulic working chamber, and in a central position, a holding of the pressure medium pressure in both pressure chambers of each hydraulic working chamber of the device. When the internal combustion engine has been switched off and the volume of the A pressure chamber of each hydraulic working chamber is minimized, the element fixed to the camshaft can be mechanically coupled to the element fixed to the crankshaft in a preferred basic position for starting the internal combustion engine, coupling be achieved by an additional locking element arranged on the element fixed to the camshaft or on the element fixed to the crankshaft, which locking element can be displaced by a spring element into a coupling position within a complementary reception in the element fixed to the crankshaft or in the element fixed to the camshaft. This complementary reception of the locking element is hydraulically connected to the pressure medium supply to the volume-minimized A pressure chamber of at least one hydraulic working chamber of the device, so that, upon pressurization of the volume-minimized A pressure chamber of each hydraulic working chamber during starting of the internal combustion engine, the reception of the locking element is likewise pressurized and the locking element is displaced hydraulically, against the force of its spring element, into an uncoupling position.
This shock-like pressurization of the volume-minimized A pressure chamber generally effected by a maximum energization of the solenoid has proved to be a drawback in practice because the sudden rise of pressure in all the volume-minimized A pressure chambers causes a bracing moment to act on the locking element which leads to a jamming of the locking element in its coupling position particularly when the mean moment of drag of the camshaft acts in the same direction as the bracing moment and/or when the time for building up the bracing moment is shorter than the time for displacing the locking element into its uncoupling position. As a consequence, a relative rotation between the element fixed to the camshaft and the element fixed to the crankshaft through a defined angle given by the microprocessor is no longer possible, so that, for example, in the case of inlet-side camshaft adjusting devices there are performance deficits of the internal combustion engine, and in the case of outlet-side camshaft adjusting devices, the internal combustion engine has higher emission values.
The object of the invention is therefore to provide a method of actuating a device for varying the valve timing of an internal combustion engine, particularly a camshaft adjusting device having a hydraulically releasable start locking, which method enables a reliable prevention of a jamming of the locking element in its coupling position even when a bracing moment resulting from a pressurization and a mean moment of drag of the camshaft impede movement of the locking element in the same direction.
The invention achieves this object in a device by the fact that the solenoid of the hydraulic valve is operated on starting of the internal combustion engine following a method of actuation of the instant invention.
According to this actuating method of the invention, the solenoid of the hydraulic valve is at first subjected, for a defined period of time, to a sudden energization with a current of an intensity lower than that required for the central position of the valve piston of the hydraulic valve, so that, to begin with, only the B pressure chamber of each hydraulic working chamber of the device is pressurized with a pressure medium pressure by which the element fixed to the camshaft is held hydraulically in its basic position. This is followed, during a further defined period of time, by a gradual increase of the energization of the solenoid to a current intensity that is higher than that required for the central position of the valve piston of the hydraulic valve, so that, when the central position of the valve piston has been passed, even the volume-minimized A pressure chamber of each hydraulic working chamber of the device, and thus also the reception of the locking element is pressurized with a pressure medium pressure. During the time of gradual increase of energization, or if desired, even following this, the microprocessor checks whether the element fixed to the camshaft has been angularly displaced relative to the element fixed to the crankshaft out of its basic position and the locking element has therefore taken its uncoupling position. If the microprocessor determines that the element fixed to the camshaft is still in its basic position, that is to say that the locking element has remained in its coupling position, the routine consisting of the sudden energization, the gradual increase of current intensity and the position check by the microprocessor is broken off and the routine is repeated till the microprocessor registers that the element fixed to the camshaft has been angularly displaced relative to the element fixed to the crankshaft out of its basic position and, thus, that the locking element has come into its uncoupling position. When such an angular displacement of the element fixed to the camshaft has been determined by the microprocessor, the routine is likewise broken off and, as a last step, the solenoid of the hydraulic valve is energized with a current defined by the microprocessor that is required for reaching or maintaining an adjusted angular position of the element fixed to the camshaft relative to the element fixed to the crankshaft.
According to an advantageous feature of the invention, it is proposed to actuate the solenoid of the hydraulic valve preferably with a pulse width modulated voltage control so that the energization of the solenoid at the start of the internal combustion engine preferably begins with a sudden rise from 0% to 5% below the intensity required for the central position of the valve piston of the hydraulic valve and, for passing the central position of the valve piston, is then raised relatively slowly from 5% below to 5% above the intensity required for the central position of the valve piston of the hydraulic valve. When an angular displacement of the element fixed to camshaft out of its basic position has been determined by the microprocessor, the pulse width modulated voltage control is set preferably so that the energization of the solenoid of the hydraulic valve again increases suddenly to a value between 5% above the intensity required for the central position of the valve piston of the hydraulic valve and 100% of the maximum intensity, depending on which adjusted angular position of the element fixed to the camshaft relative to the element fixed to the crankshaft is to be reached next. However, in place of a pulse width modulated voltage control with this setting, it is also possible to realize the energization of the solenoid with a current regulation of a type, known per se.
According to a further feature of the method of the invention, it is finally proposed that the sudden actuation of the solenoid of the hydraulic valve both before and after passing of the central position of the valve piston of the hydraulic valve should be effected in a time period of between 5 ms and 10 ms, while the phase of passing the central position of the valve piston of the hydraulic valve is preferably limited to a time period in the range of 100 ms to 300 ms. It has been determined in practice that, in most cases, the locking element reliably takes its uncoupling position already after one, or at the latest after two completed routines and any desired adjusted angular position between the crankshaft and the camshaft of the internal combustion engine can be realized. The purpose of the brief pressurization of the B pressure chamber of each hydraulic working chamber at the beginning of the routine is to bring the element fixed to the camshaft that, due to the moment of drag of the camshaft, has been rotated through an angle (up to 1xc2x0 crankshaft angle) corresponding to the operational play of movement of the locking element and that may be jamming the locking element, back into a position in which the locking element can move freely for the following pressurization of the A pressure chamber of each hydraulic working chamber of the device.
The method of the invention for actuating a device for varying the valve timing of an internal combustion engine, particularly a camshaft adjusting device having a hydraulically releasable start locking thus permits, in contrast to prior art actuating methods for devices of a similar type, a reliable prevention of a jamming of the locking element in its coupling position during the starting of the internal combustion engine even when a bracing moment resulting from the pressurization of the volume-minimized A pressure chamber of each hydraulic working chamber of the device and a mean moment of drag of the camshaft impede movement of the locking element in the same direction. The energization of the solenoid of the hydraulic valve according to the invention comprising an initial brief pressurization. of the B pressure chamber of each hydraulic working chamber followed by a slow passing of the central position of the valve piston of the hydraulic valve and the pressurization of the A pressure chamber of each hydraulic working chamber guarantees a reliable movement of the locking element into its uncoupling position and, thus also, a reliable releasing of the start locking of the device. In this way, already immediately after the starting of the internal combustion engine, relative rotations between the element fixed to the camshaft and the element fixed to the crankshaft into defined angular positions given by the microprocessor are possible, and negative consequences such as reduced performance or increased emission of the internal combustion engine are excluded.